Adducts of streptokinase and streptokinase-plasminogen complex with polyethylene glycol and the pluronic polyol F38 were prepared with the aim of developing a non-sensitizing form of streptokinase. The adducts were much less antigenic than the native protein or protein complex. Full amidolytic activity was retained when catalysis was measured with a chromogenic substrate. However, dissolution of fibrin clots by the adducts was greatly reduced from that obtained with native streptokinase.